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Abstract:

The invention relates to a method of preparing a mixture of biofuels
comprising fatty acid esters and at least one mixture of glycerol ethers
from fatty substances that may contain free fatty acids and ethanol
comprising: a) a step of transesterification of a vegetable or animal oil
by ethanol in the presence of a catalyst based on at least one alkali
metal salt or ammonium heteropolyacid salt characterized by differential
heat of absorption of ammonia which is greater than or equal to 150
kJ/mol, stable at T>150° C., in order to obtain fatty acid
esters and glycerol, and, b) a step of etherification of the glycerol
formed during step a) by the alcohol used in step a) in the presence of
the catalyst from step a) in order to obtain at least one ether of the
glycerol, said steps a) and b) taking place simultaneously, in one and
the same reactor.

Claims:

1. A method for preparing a mixture of biofuels comprising fatty acid
esters and at least one mixture of glycerol ethers from fatty substances
and ethanol, comprising: a) a step of transesterification of a vegetable
or animal oil by ethanol in the presence of a catalyst based on at least
one alkali metal or ammonium heteropoly acid salt characterized by a
differential heat of absorption of ammonia greater than or equal to 150
kJ/mol, in order to obtain fatty acid esters and glycerol; and b) a step
of etherification of the glycerol formed during step a) by the ethanol
used in step a) in the presence of the catalyst from step a) in order to
obtain at least one glycerol ether, said steps a) and b) taking place
simultaneously, in one and the same reactor.

2. The method as claimed in claim 1, characterized in that the glycerol
ethers are chosen from the monoethers and the ethers of glycerol.

3. The method as claimed in claim 1, in which the molar ratio between the
alcohol and the vegetable or animal oil is between 1 and 50.

4. The method for the etherification of glycerol by ethanol comprising a
step of reaction between glycerol and ethanol in the presence of a
catalyst based on at least one alkali metal or ammonium heteropoly acid
salt characterized by a differential heat of adsorption of ammonia
greater than or equal to 150 kJ/mol, stable at T>150.degree. C.

5. The method as claimed claim 1, characterized in that the catalyst
based on at least one alkali metal or ammonium heteropoly acid salt has a
differential heat of absorption of ammonia greater than or equal to 170
kJ/mol.

6. The method as claimed in claim 1, characterized in that the heteropoly
acid salt is chosen from the salts of the heteropoly acids of general
formula: HkXlM.sub.mOn.xH2O in which: X represents a
heteroatom chosen from the group constituted by the following elements:
P, Si, Ge, B or As; M represents a peripheral metallic element chosen
from the group constituted by W, Mo or V; l is the number of heteroatoms
and represents 1 or 2; k is the number of hydrogen atoms and is between 1
and 10; m is the number of peripheral metallic atoms W, Mo, V and is
between 1 and 20; n is the number of oxygen atoms and is between 2 and
62; x is the number of molecules of water of hydration and is between 0
and 40.

7. The method as claimed in claim 1, characterized in that the heteropoly
acid salt is chosen from the group of the salts of heteropoly acids
chosen from the group constituted by H3PW12O.sub.40.24H2O,
H4SiW12O.sub.40.24xH2O,
H6P2W18O.sub.62.24H2O,
H5BW12O.sub.40.30H2O,
H5PW10V2O.sub.40.xH2O,
H3PMo12O.sub.40.28H2O,
H4SiMO12O.sub.40.13H2O,
H3PMo6V6O.sub.40.xH2O or
H5PMo10V2O.sub.40.xH2O.

8. The method as claimed in claim 7, characterized in that the salt is
chosen from the alkali metal salts Cs.sup.+, K.sup.+ or Rb.sup.+ or
ammonium salts (NH.sub.4.sup.+).

9. The method as claimed in claim 1, characterized in that it is carried
out at a temperature between 100 and 300.degree. C., and at a pressure
between 5 and 100 bar.

10. The use of a catalyst based on at least one alkali metal or ammonium
heteropoly acid salt characterized by a differential heat of absorption
of ammonia greater than or equal to 150 kJ/mol in order to carry out an
etherification of glycerol by ethanol.

11. The use of a catalyst based on at least one alkali metal or ammonium
heteropoly acid salt, stable at 200.degree. C., for simultaneously
carrying out: a transesterification of a vegetable or animal oil by
ethanol in order to obtain ethyl esters of fatty acids and glycerol; and
an etherification of said glycerol by ethanol, in which the catalyst
based on at least one alkali metal or ammonium heteropoly acid salt is
characterized by a differential heat of absorption of ammonia greater
than 150 kJ/mol.

12. The use as claimed in claim 10, in which the heteropoly acid is
chosen from solid heteropoly acids of general formula:
HkXlM.sub.mOn.xH2O in which: X represents a
heteroatom chosen from the group constituted by the following elements:
P, Si, Ge, B or As; M represents a peripheral metallic element chosen
from the group constituted by W, Mo or V; l is the number of heteroatoms
and represents 1 or 2; k is the number of hydrogen atoms and is between 1
and 10; m is the number of peripheral metallic atoms W, Mo, V and is
between 1 and 20; n is the number of oxygen atoms and is between 2 and
62; x is the number of molecules of water of hydration and is between 0
and 40.

13. A biofuel comprising ethyl esters of fatty acids and a mixture of
monoethyl ethers and diethyl ethers of glycerol.

14. The method as claimed in claim 4, characterized in that the catalyst
based on at least one alkali metal or ammonium heteropoly acid salt has a
differential heat of absorption of ammonia greater than or equal to 170
kJ/mol.

15. The method as claimed in claim 4, characterized in that the
heteropoly acid salt is chosen from the salts of the heteropoly acids of
general formula: HkXlM.sub.mOn.xH2O in which: X
represents a heteroatom chosen from the group constituted by the
following elements: P, Si, Ge, B or As; M represents a peripheral
metallic element chosen from the group constituted by W, Mo or V; l is
the number of heteroatoms and represents 1 or 2; k is the number of
hydrogen atoms and is between 1 and 10; m is the number of peripheral
metallic atoms W, Mo, V and is between 1 and 20; n is the number of
oxygen atoms and is between 2 and 62; x is the number of molecules of
water of hydration and is between 0 and 40.

16. The method as claimed in claim 4, characterized in that the
heteropoly acid salt is chosen from the group of the salts of heteropoly
acids chosen from the group constituted by
H3PW12O.sub.40.24H2O,
H4SiW12O.sub.40.24xH2O,
H6P2W18O.sub.62.24H2O,
H5BW12O.sub.40.30H2O,
H5PW10V2O.sub.40.xH2O,
H3PMo12O.sub.40.28H2O,
H4SiMO12O.sub.40.13H2O,
H3PMo6V6O.sub.40.xH2O or
H5PMo10V2O.sub.40.xH2O.

17. The method as claimed in claim 16, characterized in that the salt is
chosen from the alkali metal salts Cs.sup.+, K.sup.+ or Rb.sup.+ or
ammonium salts (NH.sub.4.sup.+).

18. The method as claimed in claim 4, characterized in that it is carried
out at a temperature between 100 and 300.degree. C. and at a pressure
between 5 and 100 bar.

19. The use as claimed in claim 11, in which the heteropoly acid is
chosen from solid heteropoly acids of general formula:
HkXlM.sub.mOn.xH2O in which: X represents a
heteroatom chosen from the group constituted by the following elements:
P, Si, Ge, B or As; M represents a peripheral metallic element chosen
from the group constituted by W, Mo or V; l is the number of heteroatoms
and represents 1 or 2; k is the number of hydrogen atoms and is between 1
and 10; m is the number of peripheral metallic atoms W, Mo, V and is
between 1 and 20; n is the number of oxygen atoms and is between 2 and
62; x is the number of molecules of water of hydration and is between 0
and 40.

Description:

[0001] The production of methyl or ethyl ethers of fatty acids (biodiesel)
during the transesterification reaction of a fatty substance inevitably
produces glycerol. The upgrading thereof is a determining factor for the
equilibrium of the biodiesel field.

[0002] Furthermore, glycerol ethers are also potential fuel additives
which may go into the diesel fuel pool.

[0003] The most common transesterification processes use a basic
homogeneous catalysis, for example the processes described in patent
application US 2003/0032826 (University of Nebraska). The products of the
reaction must then undergo steps of neutralization, washing and
separation in order to obtain the fatty acid esters, but also the
glycerol of sufficient purity in order to be sold.

[0004] Continuous or batch processes for the transesterification of oils
by monoalcohols that require heterogeneous catalysis have appeared more
recently. Such as, for example, the processes described in patent
applications U.S. Pat. No. 5,908,946 (IFP) or US 2004/0112212 (IFP), in
which the glycerol obtained, of higher purity, is decanted from the
reaction medium then removed.

[0005] The processes for transesterification by ethanol are less
widespread. Transesterification by ethanol is generally less effective.
In particular, via basic catalysis, transesterification by ethanol is
slower than with methanol, methanol being more acidic than ethanol.
Furthermore, one of the reasons mentioned relates to the better solvent
power of ethanol which is responsible for the poor separation of the
glycerin from the reaction medium. Furthermore, since the
transesterification is a balanced reaction, the solubilization of the
glycerol in the reaction medium may have the result of limiting the
progress of the reaction. This high solvent power of the ethanol
furthermore has a drawback at the end of the reaction; it is more
difficult to separate the glycerin from the reaction medium by
decantation, see US 2007/0112212 (IFP).

[0006] To overcome this major difficulty, two-step processes are thus
proposed, for example in US 2007/0066838 A1 (IFP), in order to prepare
ethyl esters of linear monocarboxylic acids from vegetable or animal oil
comprising a transesterification by methanol in a first step then a
second transesterification step in which the reaction medium produced is
reacted with ethanol.

[0007] The transesterification of vegetable oil by alcohols in the
presence of heteropoly acids has also been described by V. V. Bokade et
al., Trans IChemE, Part B, Process safety and Environmental Protection
(2007), 85 (B5), 372-377. The authors studied the transesterification
reaction of a vegetable oil with a supported heteropoly acid catalyst.
Screening of various catalysts enabled them to distinguish a specific
catalyst that gives good conversion yields with methanol: 10% of
dodecatungstophosphoric acid on clay. This catalyst was then studied for
the transesterification of an oil with various alcohols; it is noticed
from this study (table 5) that the yields are higher with methanol than
with ethanol (respectively 84 and 80% conversion). The authors also
suggest that it might be possible to continue the reaction and to form
glycerol ethers.

[0008] More recently, the transesterification of rapeseed oil with ethanol
in the presence of strong Bronsted acid catalysts (heteropoly acids of
Keggin structure) has been described by N. Essayem et al. Appl. Catal. A:
General 330 (2007) 69-76. The separation of the glycerol is not however
addressed in this article. The reaction described has a yield of less
than 55%.

[0009] The glycerol may be upgraded, for example as a synthesis
intermediate and may be used as an emulsifier, plasticizer, solvent, etc.
Numerous studies are under way for finding new applications for glycerol,
but these depend on the cost price of the glycerol, which is a function
of its degree of purity. The economic advantage of upgrading the glycerol
as such is obvious only if the glycerol is of low cost, therefore is not
very purified.

[0010] However, the most advantageous upgrading is an upgrading of the
latter in the field of fuels or biofuels.

[0011] Glycerol ethers are potential fuel additives which may be
incorporated into the formulation of fuels. This application is even more
advantageous since European Directives will impose the use of 5.75% of
biofuels in the transport industry in 2010. International application WO
2007/061903 A1 (CPS Biofuels) and patent U.S. Pat. No. 5,308,365 (ARCO
Chemical Technology) describe fuel compositions comprising glycerol
ethers.

[0012] It is known from WO 2007/061903 A1 (CPS Biofuels) that the addition
of glycerol ethers to the bioethanol makes it possible to reduce the
vapor pressure of the fuel obtained. Furthermore, glycerol ethers may
replace conventional oxygenated additives of the MTBE type. They also
make it possible to reduce particulate emissions and then reduce the
viscosity of the biodiesel fuel. It is also reported that the presence of
the hydroxyl group of partially etherified glycerol ethers may make it
possible to incorporate small amounts of water into the fuels, which
could reduce NOx emissions.

[0013] From WO 2005/093015 (IFP), it is known that glycerol ethers make it
possible to make the glycerol soluble in the biodiesel. In this patent
application, the obtaining of a mixture of mono-, di- and triglycerol
ethers is described, the mixture being soluble in the biodiesel.

[0014] Concentrations of 1 to 20% in diesel fuels and up to 50% in
gasolenes are reported and, for example, the incorporation into the
biodiesel of the whole of a mixture of mono-, di- and tri-tert-butyl
ethers having the average composition equivalent to a di-tert-butyl
ether.

[0015] It is also known that the addition of glycerol ethers to the
biodiesel makes it possible to reduce its viscosity and its "cloud point"
(U.S. Pat. No. 6,015,440 (University of Nebraska)).

[0016] U.S. Pat. No. 6,015,440 (University of Nebraska) and the
international application WO 2005/093 015 (IFP) report the etherification
of glycerol with isobutylene via an acid catalyst. The manufacture of
t-butyl ethers of glycerol from tert-butanol is also described.
Furthermore, international application WO 2007/113 776 (Procter & Gamble)
describes a process for converting glycerol to alkyl glycerol ethers
catalyzed by Lewis or Bronsted acids. More specifically, the
etherification of glycerol by methanol or isopropanol in the presence of
an Amberlyst resin is reported.

[0017] Biodiesel production processes have been described that use two
separate steps, including a step of etherification of the glycerol. For
example, in US 2007/0260078 (Ramanath et al.), the first step is a
transesterification reaction of a vegetable oil by an alcohol, in the
presence of a double metal cyanide catalyst, the reaction medium is then
cooled and reacted with the alcohol in the presence of an Amberlyst
catalyst. The reaction gives rise to the formation of a biodiesel
comprising ethyl esters of fatty acids and triethyl ethers of glycerol.
However, Amberlyst catalysts are very sensitive to temperature and
degrade at high temperatures, furthermore this process has two steps
which is a drawback for an industrial application.

[0018] Moreover, it has been demonstrated (see WO 2007/061903) that the
presence of the hydroxyl group, therefore of monoethers or diethers of
glycerol, was more advantageous. As described above, it was suggested in
V. V. Bokade et al., that it could be possible to continue the reaction.
However, this suggestion is not demonstrated in the least. Furthermore,
the reaction described in this publication has a significant
monoglyceride and diglyceride selectivity (35% for the reaction with
ethanol), however to continue the reaction a lower amount of
monoglyceride and diglyceride is required.

[0019] This transesterification process also has the drawback of using
supported heteropoly acid catalysts which are leachable catalysts and the
activity and the strength of which depend on the nature of the support
and on the acid loading on the support.

[0020] The objective of the present invention is a method for producing a
biodiesel by transesterification and etherification reaction, in a single
step, of a vegetable oil with ethanol.

[0021] Another objective of the present invention is a method for
producing a biodiesel that makes it possible to upgrade the secondary
products formed and in particular glycerol.

[0022] Another objective of the present invention is a biofuel comprising
ethyl esters of fatty acids and a mixture of ethyl ethers of glycerol.

[0023] The inventors have surprisingly discovered that the family of
heterogeneous strong Bronsted acid catalysts (heteropoly acid salts)
makes it possible to transesterify an oil and to simultaneously produce
ethers of glycerol without isolating the intermediate glycerol.
Surprisingly, the inventors have discovered that it was possible to
upgrade all the secondary products that are formed during the reaction
and in particular to etherify the glycerol, in a single step, at the same
time as the transesterification reaction.

[0024] The transesterification of oils by an alcohol generates in situ
glycerol in the reaction medium, which is converted to alkyl ethers of
glycerol in the presence of a heterogeneous acid catalyst capable of
catalyzing the transesterification and etherification reactions by the
same alcohol.

[0025] The use of heterogeneous acid catalysis compared to the
conventional methods of basic homogeneous catalysis exhibits a major
advantage in the upgrading of oils which are potentially acidic, for
example the wasted oils which may have a high content of free acids and
which may contain greater or smaller traces of water. Indeed, when water
is capable of adversely affecting the rate of the reaction this will not
be a major problem as in the case of a conventional basic homogeneous
catalysis in which the presence of water promotes the hydrolysis of the
oil to free acids, the latter, in the presence of the alkali metal
cations of the homogeneous base, form soaps which produce emulsions in
the reaction medium, etc. But also, a basic solid catalyst will be
capable of exhibiting deactivation by adsorption of the free fatty acids
at its surface in the event of oils having a high acidity index being
used.

[0026] Since the fatty acid esters and the glycerol ethers are components
of biofuels, the whole of a fatty substance may be converted to diesel
fuel without having to separate and purify the glycerol, which is an
enormous advantage in terms of cost compared to the methods of the prior
art.

[0027] Indeed, the method according to the invention makes it possible to
eliminate the expensive steps of isolating and of purifying the glycerol.

[0028] Furthermore, the glycerol ethers formed are the most favorable with
respect to the reduction of NOx emissions.

[0029] Furthermore, by simply considering that the glycerol produced by
transesterification of the oils represents 10% by weight of the fatty
acid esters produced, the method according to the invention enables an
increase in the yield of more than 15% by weight.

[0030] Moreover, the use of the same catalyst for carrying out the
transesterification and the etherification on the one hand, and of the
same reactant, namely an alcohol, for carrying out both reactions, also
represents an economic advantage. It is not necessary to use another
reactant of olefin type for synthesizing the glycerol ethers.

[0031] Furthermore, the solid catalyst used does not undergo the leaching
observed with the catalysts based on supported acids, withstands washing
and can therefore be easily isolated from the biofuel formed.

[0032] Furthermore, since the method does not require a glycerol
separation step, it enables an alcohol having a high solvent power, such
as ethanol, to be used which is advantageous since this alcohol is
"bio-sourced", is a by-product of agricultural waste recycling processes,
is available at low cost and is not toxic compared to methanol.

[0033] Finally, the method enables all the secondary products which may be
formed during the reaction to be upgraded.

[0034] The present invention relates to a method for preparing a mixture
of biofuels comprising fatty acid esters and at least one mixture of
glycerol ethers from fatty substances and ethanol, comprising: [0035] a)
a step of transesterification of a vegetable or animal oil by ethanol in
the presence of a catalyst based on at least one alkali metal or ammonium
heteropoly acid salt characterized by a differential heat of absorption
of ammonia greater than or equal to 150 kJ/mol, in order to obtain fatty
acid esters and glycerol; and [0036] b) a step of etherification of the
glycerol formed during step a) by the ethanol used in step a) in the
presence of the catalyst from step a) in order to obtain at least one
glycerol ether, [0037] said steps a) and b) taking place simultaneously,
in one and the same reactor.

[0038] In one embodiment, the catalyst based on at least one alkali metal
or ammonium heteropoly acid salt is insoluble in the reaction medium and
the biofuel obtained.

[0039] In one embodiment, the catalyst is a catalyst based on at least one
alkali metal heteropoly acid salt.

[0040] In one embodiment, the catalyst is a catalyst based on at least one
ammonium heteropoly acid salt.

[0041] In one embodiment, the differential heat of absorption of ammonia
is greater than 170 kJ/mol.

[0042] In one embodiment, the differential heat of absorption of ammonia
is greater than 190 kJ/mol.

[0043] The glycerol that acts as the reactant for step b) corresponds to a
product from step a). It is a non-isolated intermediate product. The
method according to the invention advantageously makes it possible not to
isolate and purify the glycerol in order to convert it to ethyl ether of
glycerol (component of the biofuel).

[0044] The expression "steps a) and b) take place simultaneously" means
that the two reactions take place simultaneously in the reaction medium
("one-pot" reaction), the glycerol formed during step a) being converted
to glycerol ether as soon as it is formed. The inventors have
surprisingly discovered that, generally, the reaction medium obtained at
the end of the process may be free of glycerol if the conversion is
continued by means known to the person skilled in the art, namely
increase of the reaction time, of the mass of catalyst or by
recirculation of the reaction medium.

[0045] The term "glycerol", also known as "glycerin", denotes
1,2,3-propanetriol. The glycerol may be pure glycerol, but also glycerol
comprising impurities, especially water, inorganic salts (chloride,
phosphate, sulfate, acetate), organic compounds (fatty acids, fatty acid
esters, derivatives of glycerides, etc.). These impurities may represent
from 5 to 95% by weight relative to the weight of the glycerol. The
glycerol may in particular be the crude glycerol obtained by
transesterification of vegetable or animal oils within the context of
biodiesel production. The expression "crude glycerol" denotes the
glycerol obtained by simple decantation of the reaction medium at the end
of the transesterification of vegetable or animal oils.

[0046] The expression "etherification of glycerol" denotes the chemical
reaction which makes it possible to convert glycerol to glycerol ethers.

[0047] The expression "glycerol ethers" denotes the mono-, di- and
triethers of glycerol. In the case of mono- and diethers of glycerol, the
ether function(s) may be located at any one of the 1, 2 or 3 position(s).
The reaction for formation of the various glycerol ethers follows a
successive path: the monoether then the diether and the triether of
glycerol are produced: it is possible to promote the formation of the
diether and triether by increasing the reactants/catalyst contact time
(for example by increasing the mass of catalyst or the reaction time) or
it is possible to recirculate the product of the reaction in order to
increase the glycerol conversion and move towards the production of
triethers of glycerol.

[0048] The mixture of glycerol ethers obtained simply has to be soluble in
the biodiesel or in the other fuels such as diesel (from oil) or gasoline
(or even bioethanol) into which it will be added.

[0049] In one preferred embodiment, the expression "glycerol ethers" is
understood to mean the monoethers and diethers of glycerol.

[0051] The expression "heteropoly acid" is understood to mean a compound
constituted of hydrogen and oxygen with metallic elements (such as
tungsten, molybdenum or vanadium) and non-metallic elements, generally
from the p block of the Periodic Table (such as silicon, phosphorus or
arsenic).

[0052] In one embodiment, the invention relates to a method, characterized
in that the glycerol ethers are chosen from the monoethers and diethers
of glycerol.

[0053] In one embodiment, the invention relates to a method, in which the
molar ratio between the ethanol and the vegetable or animal oil is
between 1 and 50, preferentially between 3 and 20.

[0054] In one embodiment, the invention relates to a method for the
etherification of glycerol by ethanol comprising a step of reaction
between glycerol and ethanol in the presence of a catalyst based on at
least one alkali metal or ammonium heteropoly acid salt characterized by
a differential heat of adsorption of ammonia greater than or equal to 150
kJ/mol.

[0055] In one embodiment, the invention relates to methods, characterized
in that the catalyst based on at least one alkali metal or ammonium
heteropoly acid salt has a differential heat of absorption of ammonia
greater than or equal to 170 kJ/mol, preferably greater than or equal to
190 kJ/mol.

[0056] Among the alkali metal or ammonium heteropoly acid salts, use may
advantageously be made of an alkali metal or ammonium salt of a solid
heteropoly acid having the general formula:

HkXlM.sub.mOn.xH2O

[0057] in which: [0058] X represents a heteroatom chosen from the group
constituted by the following elements: P, Si, Ge, B or As; [0059] M
represents a peripheral metallic element chosen from the group
constituted by W, Mo or V; [0060] l is the number of heteroatoms and
represents 1 or 2; [0061] k is the number of hydrogen atoms and is
between 1 and 10; [0062] m is the number of peripheral metallic atoms W,
Mo, V and is between 1 and 20; [0063] n is the number of oxygen atoms and
is between 2 and 62; [0064] x is the number of molecules of water of
hydration and is between 0 and 40, preferably between 6 and 30.

[0065] In one embodiment, the salts of solid, strong Bronsted acid
heteropoly acids are chosen from the group constituted by the salts of
the heteropoly acids chosen from the group constituted by
H3PW12O40.24H2O,
H4SiW12O40.24xH2O,
H6P2W18O62.24H2O,
H5BW12O40.30H2O,
H5PW10V2O40.xH2O,
H3PMo12O40.28H2O,
H4SiMO12O40.13H2O,
H3PMo6V6O40.xH2O or
H5PMo10V2O40.xH2O.

[0066] The use of a heteropoly acid in salt form has numerous advantages,
in particular from an industrial viewpoint, they make it possible, on the
one hand, unlike supported heteropoly acids (used especially by Bokade et
al.) to avoid any problem of leaching of the active phase. Moreover,
unlike supported heteropoly acids, the activity of the heteropoly acid
salts does not depend on the support or on the acid loading on the
support. In one embodiment, the salts are alkali metal salts chosen from
Cs.sup.+, K.sup.+ or Rb.sup.+, or ammonium (NH4.sup.+) salts.

[0067] In one embodiment, the salt is Cs.sup.+.

[0068] In another embodiment, the salt is a K.sup.+.

[0069] In another embodiment, the salt is an Rb.sup.+.

[0070] In a last embodiment, the salt is an ammonium (NH4.sup.+)
salt.

[0071] In the case of the etherification process, these catalysts
specifically make it possible to observe conversions of greater than 40%.

[0072] The expression "differential heat of adsorption of ammonia" denotes
the molar heat released by the adsorption of infinitesimal doses of
ammonia, at constant temperature, on the catalyst initially under vacuum
in a Tian-Calvet calorimeter.

[0073] The values of the differential heats of adsorption of ammonia
correspond to the value of the plateau of the curve representing the
variation of the differential heats (Q diff kJ.mol-1) as a function
of the amount of ammonia adsorbed if the acidic solid has homogeneous
sites in force. If the differential heats decrease with the ammonia
coverage, the value considered is the average of the differential heats
of adsorption at 50% ammonia coverage.

[0074] The average values obtained for the acid catalysts are collated in
the following table:

[0078] The expression "fatty substances" is understood to mean natural
fatty substances of any origin.

[0079] The expression "vegetable or animal oil" denotes oil of animal or
vegetable origin, such as microalgae oil, Pongamia pinnata (or Karanja)
oil, Jatropha oil, palm oil, sunflower oil, rapeseed oil, almond oil,
arachis oil, coconut oil, linseed oil, corn oil, olive oil, grapeseed
oil, castor oil, sesame oil or mustard oil, but also wasted oils that are
rich in free acids. These oils contain or are constituted of acyl
glycerols, also known as glycerides, which are esters of fatty acids and
of glycerol. There are three subclasses of acyl glycerols:
monoglycerides, diglycerides and triglycerides. The prefixes mono, di,
and tri are used according to whether the esterification relates to 1, 2
or 3 hydroxyl groups of the glycerol.

[0080] The expression "transesterification of the vegetable or animal oil
by an alcohol" denotes the chemical reaction of the triglycerides with an
alcohol in the presence of the catalyst in order to obtain esters of
fatty acids and glycerol.

[0081] The expression "etherification of glycerol by an alcohol" denotes
the reaction of glycerol and an alcohol in the presence of catalyst to
obtain at least one glycerol ether, which may be a monoether, diether or
triether of glycerol. Generally, a mixture of these ethers is obtained.

[0082] In one embodiment of the methods according to the invention, the
molar ratio between the ethanol and the vegetable or animal oil is
between 1 and 50, in particular between 3 and 20, for example 4, 6, 12 or
18.

[0083] Indeed, these molar ratios make it possible to observe conversions
of greater than 80% or even greater than 95% for step a), and of the
order of 50% for step b).

[0084] In one embodiment, the methods are carried out at a temperature
between 100 and 300° C., especially 150 to 250° C., in
particular around 200° C., and at a pressure between 5 and 100
bar, especially 10 to 75 bar, in particular 10 to 50 bar, more
particularly between 20 and 30 bar.

[0085] These reaction conditions are particularly suitable for
implementing the methods according to the invention, in particular the
etherification of glycerol by ethanol which is energetically demanding:
it requires the use of a catalyst of alkali metal or ammonium heteropoly
acid salt type at a reaction temperature of around 200° C. This
temperature is considerably greater than the maximum operating
temperature of acidic resins of Amberlyst type which is below 150°
C. The use of such catalysts is advantageous because they are stable at
these high temperatures, unlike other catalysts, such as the acidic
resins of Amberlyst type. Furthermore, these catalysts are more reactive:
by way of comparison, at 85° C. the cesium heteropoly acid salt is
4 times more active than Amberlyst 15 in relation to its more energetic
sites.

[0086] According to a second aspect, the present invention relates to the
use of a catalyst based on at least one alkali metal or ammonium
heteropoly acid salt in order to carry out an etherification of glycerol
by ethanol, in which the catalyst based on at least one alkali metal or
ammonium heteropoly acid salt is characterized by a differential heat of
absorption of ammonia greater than 150 kJ/mol and stable at a temperature
of 200° C.

[0087] In one embodiment, the catalyst is a catalyst based on at least one
alkali metal heteropoly acid salt.

[0088] In one embodiment, the catalyst is a catalyst based on at least one
ammonium heteropoly acid salt.

[0089] The invention also relates to the use of a catalyst based on at
least one alkali metal or ammonium heteropoly acid salt, for
simultaneously carrying out: [0090] a transesterification of a vegetable
or animal oil by ethanol in order to obtain ethyl esters of fatty acids
and glycerol; and [0091] an etherification of said glycerol by ethanol,
[0092] in which the catalyst based on at least one alkali metal or
ammonium heteropoly acid salt is characterized by a differential heat of
absorption of ammonia greater than 150 kJ/mol, stable at a reaction
temperature of 200° C.

[0093] In one embodiment, the catalyst is a catalyst based on at least one
alkali metal heteropoly acid salt.

[0094] In one embodiment, the catalyst is a catalyst based on at least one
ammonium heteropoly acid salt.

[0095] According to another aspect, the invention relates to a biofuel
comprising ethyl esters of fatty acids and a mixture of ethyl ethers of
glycerol.

[0096] In one embodiment, the invention relates to a biofuel comprising a
mixture of monoethyl ethers and diethyl ethers of glycerol.

[0097] In one embodiment, said biofuel also comprises ethanol.

[0098] The invention will be described in greater detail by means of the
following examples given by way of illustration.

COUNTER EXAMPLE 1

Etherification of Glycerol by Tert-Butanol or Ethanol in the Presence of
Amberlyst 35

[0099] The reaction conditions were the following. The catalyst was
Amberlyst A35 (m=0.39 g). 0.0275 mol of glycerol was used. The [ethanol
or tert-butanol]/glycerol molar ratio was 4. The reaction time was 3
hours.

[0100] The results appear in table 1.

[0101] The conversion is calculated according to the following equation:

100×(Glyo-Glyf)/Glyo

[0102] in which Gly represents the amount of glycerol, Glyo the
amount of glycerol at the start of the reaction and Glyf the amount
of glycerol at the end of the reaction. The selectivities and molar
yields of glycerol derivatives are calculated as follows:

[0103] Conversion and selectivity of the reaction for etherification of
glycerol by tert-butanol or ethanol catalyzed by Amberlyst A35.
(alkyl=ethyl or t-butyl)

[0104] These experiments show that the etherification of glycerol by
ethanol is energetically more demanding than the etherification by
tert-butanol due to the greater acid nature of ethanol compared to the
tertiary alcohol. This example shows the difficulty in carrying out the
etherification of glycerol by ethanol with a standard etherification
catalyst, acid resins. The conversion was not able to be improved by
increasing the reaction temperatures since the acid resins are not stable
at temperatures above 150° C.

[0105] The term HPA is understood to mean H3PW12O40 and
more precisely 40% by weight of H3PW12O40 dispersed on
supports.

EXAMPLE 2

Influence of the Nature of the Catalyst in the Etherification of Glycerol
by Ethanol

[0106] The reaction conditions were the following. 0.39 g of catalyst was
used. 0.0275 mol of glycerol was used. The ethanol/glycerol molar ratio
was 4. The temperature was 200° C. The reaction time was 6 hours.

[0107] The results appear in table 3. The most active catalysts under the
conditions tested for the formation of ethyl ethers of glycerol are
HPA/SiO2, HPA/charcoal and Cs2HPW12O40.

[0108] The comparison of tables 2 and 3 shows that regardless of the
catalyst used, the etherification of glycerol by ethanol is energetically
more demanding than the etherification by tert-butanol and therefore more
difficult to carry out. The results from tables 2 and 3 also show a
variability of the activity of the supported heteropoly acids depending
on the support.

EXAMPLE 3

Reaction Between Rapeseed Oil and Ethanol in the Presence of
Cs2HPW12O40 in Order to Produce, in a Single Step, Ethyl
Esters of Fatty Acids (Biodiesel) and Glycerol Ethers (Fuel Ethers)

[0109] Tr=200° C. for 6 hours. (Tr=reaction time)

[0110] The reaction conditions were the following. 0.5 g of
Cs2HPW12O40 catalyst was used (pretreatment: 1 h under
vacuum at 200° C.). 0.2047 mol of ethanol and 0.01144 mol (which
corresponds to Trio in the equations which follow) of rapeseed oil
were used. The ethanol/ester molar ratio was 6 (the ethanol/oil molar
ratio was 18). The rate of stirring was 500 rpm. The reaction time was 6
hours. The temperature was 200° C. The autoclave was pressurized
at 17 bar under Ar (final P=30 bar).

[0111] The results appear in tables 4 and 5.

[0112] The analysis of the derivatives of glycerol is expressed in a
similar manner to that of the preceding examples. The analysis of the
fatty products present at the end of the reaction is expressed according
to the following equations.

Reaction Between Rapeseed Oil and Ethanol in the Presence of
Cs2HPW12O40 in Order to Produce, in a Single Step, Ethyl
Esters of Fatty Acids (biodiesel) and Glycerol Ethers (fuel ethers)

[0114] Tr=85° C. for 5 h, then Tr=200° C. for 6 h.

[0115] The reaction conditions were the following. 0.5 g of
Cs2HPW12O40 catalyst was used (pretreatment: 1 h under
vacuum at 200° C.). 0.2051 mol of ethanol and 0.01138 mol (which
corresponds to Trio in the equations which follow) of rapeseed oil
were used. The ethanol/ester molar ratio was 6 (the ethanol/oil molar
ratio was 18). The rate of stirring was 500 rpm. The temperature was
85° C. for 5 hours then 200° C. for 6 hours. The autoclave
was pressurized at 17 bar under Ar (final P=30 bar).

Reaction Between Sunflower Oil and Ethanol in the Presence of
Cs2HPW12O40 in Order to Produce, in a Single Step, Ethyl
Esters of Fatty Acids (Biodiesel) and Glycerol Ethers (Fuel Ethers)

[0117] Tr=85° C. for 5 h, then Tr=200° C. for 6 h.

[0118] The reaction conditions were the following. 0.5 g of
Cs2HPW12O40 catalyst was used (pretreatment: 1 h under
vacuum at 200° C.). 0.2052 mol of ethanol and 0.01138 mol (which
corresponds to Trio in the equations which follow) of sunflower oil
were used. The ethanol/ester molar ratio was 6 (the ethanol/oil molar
ratio was 18). The rate of stirring was 500 rpm. The temperature was
85° C. for 5 hours then 200° C. for 6 hours. The autoclave
was pressurized at 17 bar under Ar (final P=30 bar).